1

Structural, microstructural and electrical

properties of Lanthanum (La+3

) modified

Lead iron niobate Pb(Fe0.5Nb0.5)O3.

Thesis submitted in partial fulfillment

of the requirements for the degree of

Master of Science (M.Sc.)

in

Department of Physics

Under the academic Autonomy

National Institute of Technology, Rourkela

by

Shreenu Pattanaik

(Roll No-410PH2141)

Under the guidance of

Dr. Dillip K Pradhan

Department of Physics

National Institute of Technology, Rourkela

Rourkela-769008, Odisha, India.

2

Department of Physics

National Institute of Technology, Rourkela

Rourkela-769008, Orissa, India.

Certificate

This is to certify that the work in the thesis entitled “Structural, microstructural and electrical

properties of Lanthanum (La+3

) modified Lead iron niobate Pb(Fe0.5Nb0.5)O3." submitted by

Miss. Shreenu Pattanaik is a record of an original research work carried out by her under my

supervision and guidance in partial fulfillment of the requirements for the award of the degree of

Master of Science in Physics. Neither this thesis nor any part of it has been submitted for any

degree or academic award elsewhere.

Dr. Dillip K Pradhan

Asst. Professor

Department of Physics,

National Institute of Technology, Rourkela-769008

3

DECLARATION

I hereby declare that the work carried out in this thesis is entirely original. It was carried out by me along

with Mr Soumya Ranjan Dash at Department of Physics, National Institute of Technology, Rourkela. I

further declare that it has not formed the basis for the award of any degree, diploma, or similar title of

any university or institution.

Shreenu Pattanaik

Roll No- 410PH2141

Department of physics

National Institute of Technology

Rourkela-769008

4

Acknowledgment

I express my sincere gratitude to my supervisor Dr.Dillip K Pradhan for his

valuable guidance and support in carrying out this project. I would like to

acknowledge all the faculties of the Department of Physics for their help and

support. I extend my thanks to all my colleagues and Ph.D. scholar Mr.Satya

Narayan Tripathy for his co-operation. I would also like to thank Soumya Ranjan

Dash for his valuable help. Finally, I would like to acknowledge Director, NIT

Rourkela for permitting me to carry out this project successfully and also for

providing the essential facilities.

Shreenu Pattanaik

Department of Physics

NIT, Rourkela

Rourkela-769008

5

Contents Pages

I. Chapter-1 :Introduction (7-15)

1.1 Ferroelectric materials

1.2 Characteristics feature of the ferroelectric materials

1.3 Types of ferroelectric materials

1.4 Perovskite structure

1.5 Ferroelectric Phase Transition

1.6 Lead Iron Niobate and related materials

1.7 Materials under investigation

1.8 Objective

II. Chapter-2 :Experimental Technique (17-24)

2.1 Introduction

2.2 Solid-state reaction route

2.3 Characterization technique

III. Chapter-3 :Result and Discussion (26-37)

3.1 Structural studies

3.2 Microstructural Studies

3.3 Dielectric

3.4 Impedance studies

IV. Conclusion

V. References

6

Abstract

One series of lanthanum-modified Lead iron niobate ceramic oxides having chemical formula

Pb(1-x)Lax(Fe0.5Nb0.5)(1-x/4)O3 (x=0.00, 0.02, 0.04, 0.06, 0.08) ceramics were prepared by mixed

oxide high temperature solid state reaction route. Preliminary structural, microstructural and

electrical properties of the compounds were studied using XRD, SEM and complex impedance

spectroscopic analysis. The formations of the compounds were confirmed by XRD analysis.

From preliminary structural using the XRD data, the lattice parameters are calculated using

standard IUCR software CHECKCELL. Williamson Hall method was used for finding the

crystallite size in the samples. Microstructures/morphology of the materials was analyzed by

scanning electron microscopy. Dielectric and impedance spectroscopic properties of the

materials were studied in a wide frequency range at different temperatures. The temperature

dependent dielectric constant showed the ferroelectric to praelectric phase transition around

1150C for Pb(Fe0.5 Nb 0.5)O3. Then with increase in La concentration, there is a decrease in the

transition temperature and above 4% La concentration, the transition temperature is below the

room temperature.

7

Chapter-1

8

Introduction

1.1 Ferroelectric Materials:

Ferroelectric materials have attracted the attention of the scientific community since last few

decades for its fundamental physics and potential device applications. The phenomenon of

ferroelectricity was discovered in 1921 by Valsek in Rockchell salt [1]. A breakthrough in the

research on ferroelectric materials came in the 1950’s, leading to the widespread use of barium

titanate (BaTiO3) based ceramics in capacitor applications and piezoelectric transducer devices.

Since then, many other ferroelectric ceramics including lead titanate (PbTiO3), lead zirconate

titanate (PZT), lead lanthanum zirconate titanate (PLZT), and relaxor ferroelectrics like lead

magnesium niobate (PMN) have been invented, studied and utilized for a variety of potential

device applications. With the development of ceramic processing and thin film technology, many

new device applications have emerged. The biggest use of ferroelectric ceramics has been in the

areas such as dielectric ceramics for capacitor application, ferroelectric thin film for non-volatile

memories, piezoelectric materials. Hence lot of research work was carried out both experimental

and theoretical – in this field in the past 4 to 5 decades [2].

Ceramics are defined as the solid compounds that consist of metallic and non-metallic

elements which are formed by the application of heat and pressure. The most important

properties of the ceramics are wear-resistant, brittle, refractory, thermal insulators, electrical

insulators, nonmagnetic, oxidation resistance, and thermal shock prone and chemically stable.

The nature of bonding between the atoms and type of atoms represents the properties of the

9

materials. Two most common chemical bonds for ceramic materials are covalent and ionic

bonding. In comparison to metals, ceramics have very low electrical conductivity [3].

Most of the ceramic materials are dielectrics which possess very low electrical

conductivity but supports electrostatic field. Electrical conductivity of ceramics depends on

temperature and frequency. This is due to the reason that charge transport mechanisms are

frequency dependent and thermal energy provides the activation energy for the charge migration.

In general, ceramic materials have high dielectric strength and high dielectric constant for

potential device applications.

1.2 Characteristics feature of the ferroelectric materials

(1) Lack of center of symmetry.

(2) Ferroelectric crystals must be piezoelectric though the converse is not true.

(3) Ferroelectric materials have one or more Curie temperature.

(4) Ferroelectric transitions are structural transitions.

(5) Ferroelectric materials obey Curie-Weiss law.

(6) Ferroelectric materials exhibits hysteresis loop

The ferroelectrics are characterized by the ferroelectric hysteresis loop, i.e., the polarization P is

a double-valued function of the applied electric field E. Typical polarization – electric field

hysteresis loop displayed by ferroelectrics. As the electric field is high enough, all the

ferroelectric domains are aligned in the direction of field, the crystal becomes monodomain and

the polarization is saturated. The extrapolation of the linear portion of the curve at high field

back to the polarization axis represents the value of the spontaneous polarization Ps. When the

electric field is removed, most of the domains remain aligned and the crystal still exhibit

10

polarization. The polarization at zero fields after saturation is called remnant polarization Pr. The

remnant polarization can be removed when a field in the opposite direction is applied and

reaches a critical value. The strength of the electric field required to reduce the polarization to

zero is called the coercive field Ec.

(Figure-1Typical Ferroelectric hysteresis loop)

1.3 Types of ferroelectric materials:

Ferroelectric materials existed can be divided in to following categories:

Corner Sharing Octahedra

(1) Perovskites (ABO3)

(2) Pyrochloro (A2B2O7)

(3) Tungsten Bronze type Compounds ( (A1)2(A2)4(C)4(B1)2(B2)8O30 )

(4) Bismuth Oxide Layer Structured Ferroelectrics ((Bi2O2)+2, (An-1,Bn O3n-1)

-2)

compounds containing hydrogen bonded radicals

organic polymers

ceramic polymer composites

11

1.4 Perovskite structure

The perovskite structure, of chemical formula ABX3, consists of two distinct cation sites

(A and B) and one anion site (C). Crystals of the prototypical perovskite, CaTiO3, were first

discovered in 1839. However, halide and sulfide perovskites are also known and of

technological interest [Bennett 2009]. A-site can be monovalent, divalent or trivalent and B-site

can be pentavalant, tetravalent or trivalent .The coordination number of A-site cation is 8 to 10

and B-site cation is 6.The structures of perovskites are determined by short range attractive

(bonding) and repulsive forces between nearby ions, as well as long range electrostatic

interactions between unit cells. Composition determines the balance of these forces, and

therefore the structure. The stability of peroskite is represented by tolerance factor (t) and

determines the property of the perovskite [4].

(Figure-2 Typical Perovskite structure)

(1) t ≥ 1, for ferroelectric perovskites, BaTiO3, PbTiO3 and KNbO3 ,

(2) t < 1 for antiferroelectric perovskites PbZrO3 , NaNbO3 and BiFeO3 .

(3) t=1 corresponds to an ideal perovskite

While a t<1 Indicates tilting or rotation of the BO6 octahedral and for t>1 a displacive

distortion within the BO6 octahedral.

)(2

)(

OB

OA

RR

RRt

12

1.5 Ferroelectric Phase Transition

The transition from ferroelectric (less-symmetric or distorted structure) to paraeletric (high-

symmetric centrosymmetric) phase in most of the times is accompanied by a structural or

crystallographic phase transition. In modern language phase transition quite often referred to

symmetry breaking. The phase transition in a ferroeletric material is usually of two types

(1) Displacive phase transition

(2) Order-disorder phase transition

(Figure-3 Structural phase transition)

Displacive transitions proceed through a small distortion of the bonds (dilatational or rotational).

As the key for the ferroelectricity, Ps arises due to the non-concidence of the positive and

negative charge centers in the unit cell. Ferroelecity primarily results from the off center

displacement of the B cation in ABO3 peroskite structure leading to displacive phase transition.

This off center displacement in the peroskite ferroelerics is the result of competition between the

short range repulsion force between adjacent electron clouds of ions, which favours the non

ferroeletric centrosymmetric structure and additional bonding between B cation and oxygen ions.

Order-disorder transitions proceed through substitution between atoms possibly followed by

small atomic displacements. They are commonly found in metals and alloys and in ceramics [5].

13

Order Parameter

Most phase transitions are characterized by the appearance of some non-zero quantity in the

ordered state and the same varnishes in the disorder state. Such quantity is called order

parameter.

Normally, one encounters two type of phase transition according to order parameter.

(1) First order phase transition, characterized by the appearances of latent heat, finite change

in volume and hysteresis. (First derivative of Gibb’s free energy is discontinuous.)

(2) A second order phase transition, characterized by discontinuity in the specific heat.

(First derivative of Gibb’s free energy is discontinuous.)

It is well known that order parameter is a decreasing function of temperature and must

varnish at critical temperature. If the order parameter varnishes discontinuously at Tc, then the

transition is said to be first order while if it varnished continuously, it is called second order.

Near the Curie point or phase transition temperature, thermodynamic properties

including dielectric, elastic, optical, and thermal constants show an anomalous

behavior. This is due to the distortion in the crystal as the phase changes. The temperature

dependence of the dielectric constant above the Curie point (T>T C) in most of the ferroelectric

crystals is governed by the Curie-Weiss law:

Where, ε is the permittivity of the material, ε 0 is the permittivity of the vacuum, C is the Curie

constant and T 0 is the Curie-Weiss temperature. In general the Curie-Weiss temperature T 0 ,

is different from the Curie temperature T C . For first order transitions, T 0 <T C while for

0

0TT

C

14

second order phase transitions, T 0 =T C .

Complex perovskite type ferroelectrics with distorted cation arrangements show

DPT (diffuse phase transition) which is characterized by a broad maximum for the

temperature dependence of dielectric constant (ε) and dielectric dispersion in the transition

region. For DPT, ε follows modified temperature dependence Curie Weiss law.

where, T m is the temperature at which ε reaches maximum (ε m ), C is the modified

Curie constant and γ is the critical exponent. The γ factor explains the diffusivity of the

materials, which lies in the range 1<γ<2. In case of γ equals to unity, normal Curie–

Weiss law is followed and it shows the normal ferroelectric phase transition.

1.6 Lead Iron Niobate and related materials

Lead iron niobate Pb (Fe0.5 Nb0.5) O3 (PFN) is a lead based complex perovskite which

is of great interest for multilayer capacitors owing to its high dielectric constant discovered by

Smolenskii et al. PFN having a monoclinic structure, and a Curie temperature of 114 °C,

therefore becomes a possible candidate for making new relaxor ferroelectrics exhibiting

attractive piezoelectric properties. Recently it has been proposed that certain complex

perovskite ferroelectric compounds. A (B I B II) O 3 type, containing transition metal as one

of the B cations, exhibit magnetic ordering through indirect exchange-mechanism. It has been

studied ceramic powders, single crystal and also thin films. Raymond et al. extensively studied

)(0

mTT

C

15

the structural, thermoelectric, dieletric and impedance spectroscopic studies of lead iron niobate

[6]. Wang et al. showed the enhanced dielectric properties in 0.94PFN-0.06PT single crystals

[7]. Kumar et al. studied the enhanced electrical properties of PFN thin films. Mishra et al.

reported the dieletric relaxation and magnetic properties of the PFN ceramics [8]. Sahoo et al.

studied the effect of V5+ and Y

+3 substitutions on dieletric and ferroelectric properties of PFN

[9]. Varshney et al. reported the dieletric properties of Ba2+

modified lead iron niobate [10].

Based on the literature survey, some of the main problems in PFN based material are

(1) Difficult to synthesize single phase PFN material. For example, when synthesized using

conventional solid-state synthesis route, a pyrochlore phase ( Pb3Nb4O13 or Pb2Nb2O7

type ) with lower dielectric constant always coexists with the desired perovskite phase

in PFN ceramics[11].

(2) PFN ceramics or similar iron-doped systems, the occurrence of Fe 2+

and oxygen

vacancies originated during the sintering process increase electrical conductivity,

large frequency dispersion of dielectric constants , dielectric loss, space-charge

accumulation at the grain boundary, all of them inhibition to optimal device

performance. PFN ceramics also exhibit lower resistivity, for example, makes it almost

impossible to pole PFN ceramics to determine the piezoelectric constants as well as to

measure the polarization hysteresis characteristics[6].

Among the various methodologies available in literature, one of the important methods to solve

the above mention problems is suitable substitutions. In the present study, we have planned to

substitute La at the Pb-site of the complex perovskite PFN.

16

1.6 Materials under investigation:

Pb (1-x)Lax(Fe0.5 Nb0.5)1-x/4O3 (x=0.0, 0.02, 0.04, 0.06, 0.08)

1.7 Main Objectives:

The following will be the main objectives of the proposed work.

(1) Preparation of new complex ferroeletric compounds using high temperature solid state

reaction route.

(2) Studies of the structural & micro-structural properties of the materials.

(3) Studies of dielectric responses as a function of frequency & temperature.

(4) Studies of Impedance and conductivity studies of the sample.

17

Chapter-2

18

Experimental Technique

In this chapter, the basic principles and various experimental techniques used in this work are

briefly discussed. The characterization techniques include structural analysis, surface

morphology study and dielectric measurements.

2.1Introduction

Ceramic materials are important class of in material science because of their direct and indirect

application to solid-state device application and day to day life. Eventually synthesis of these

materials is of a greater importance to the progress of material science. There are several method

for synthesis of ceramic materials such as solid state reaction route, high energy ball milling (top

to bottom approach), soft chemical route (bottom to top approach). To achieve a qualitative

product with respect to purity, homogeneity, reactivity each method is having its own advantages

and disadvantages. In this view, solid-state reaction route is found to be easier and low cost

method by means of performance and economy.

2.2Solid State reaction route

The solid-state reaction route is the most widely used method for the preparation

of polycrystalline solids from a mixture of solid starting materials. Solids do not react together at

room temperature over normal time scales and it is necessary to heat them to much higher

temperatures The factors on which the feasibility and rate of a solid state reaction include,

reaction conditions, structural properties of the reactants, surface area of the solids,

their reactivity and the thermodynamic free energy change associated with the reaction. In

addition, the higher temperature allows some movement or flow of atoms through the solid at a

19

sufficient rate so that the desired product can eventually be obtained. A number of procedures

are used to reduce the time needed for synthesis. The starting materials are often intimately

grinded together, thus ensuring good mixing and hence increasing contact between the reacting

grains.

Experimental Details

Polycrystalline powder of Pb (1-x)Lax(Fe0.5Nb0.5)1-x/4O3 (x=0.0, 0.02, 0.04, 0.06, 0.08) were

synthesized by a conventional solid-state reaction route. The high purity oxides (PbO, Fe2O3,

Nb2O5, La2O3,) (LOBA Chemie Private Limited, Mumbai, India) of required precursors

were weighed according to the stoichiometric ratios and mixed by agate mortar and pestle for 2

hours then in wet (acetone) medium to obtain a homogeneous mixture for 4 hrs. 3% extra PbO

was taken in order to compensate lead loss at high temperature. The dried mixture was put in

alumina crucible and calcined at temperature 900 O C for 6 hours in furnace. The above heated

powder formed into a lump and it was grinded till it became fine powder. Phase formation was

checked by XRD at room temperature. The above calcined powder was mixed with 6% PVA

(polyvinyl alcohol) as a binder in mortar and pestle. The binder mixed powder was compacted

to form pellet by a hydraulic press at 6 x 10 7

kg/m 2 pressure using 10mm die set.

The sintering of the pellet sample was carried out at an optimized temperature of 1000 OC . The

sintered pallets were polished by emery paper and painted with silver paste as an electrode for

electrical measurement.

20

PbO La2O3

Phase Formation (XRD)

Sintering

Characterization

Fe2O3 Nb2O5

NO YES

Pb (1-x) La x (Fe0.5 Nb0.5)1-x/4 O3

(x=0.0, 0.02, 0.04, 0.06, 0.08)

Dry Mixing

Wet Mixing

Calcination

Grinding

Recalcination

Cold pressing

Polishing

Flow chart for the preparation of ceramic samples by a solid-state reaction technique.

Electroding

21

2.3 Characterization Technique

X-ray powder diffraction

X-ray diffraction is a (XRD) powerful non-destructive technique to characterize, analyze and

interpret the detailed structural study of the compound. It predicts the quantative phase analysis

as well as qualitative structural and microstructural analysis. X-rays are used to produce the

diffraction pattern because their wavelength λ is typically the same order of magnitude that of

the spacing d between planes in the crystal. In the present study we have recorded XRD-pattern

in PAnalytic diffractometer.

The diffraction satisfies the Bragg equation,

2dsinθ = nλ

(Figure-1 Bragg’s reflection from crystallographic planes)

22

Here d is the spacing between diffracting planes, is the incident angle, n is any integer, and λ is

the wavelength of the beam.

The information in an XRD pattern is a direct result of three things:

(1) The size and shape of the unit cells, which determine the relative positions of the

diffraction peaks.

(2) 2Atomic positions within the unit cell, which determine the relative intensities of the

diffraction peaks electron (charge density distribution)

(3) Peak broadening is related to microstructural parameters (crystallite size, r.m.s strain and

dislocation density)

It is impossible to find two different materials having same x-ray diffraction pattern.

Therefore it can be used as fingerprint to identify the material[13-14].

The determination of lattice constants from the line positions or d spacing can be found from a

general formula

Where; V = volume of the unit cell

Here a, b, c, α, β and γ are lattice parameters and h, k, l are the miller indices. The above formula is used

to calculate lattice parameters for all the compositions.

The microstructural parameters crystallite size (D) and strain can be related by Williamson-Hall equation.

β Cosθ= 4ЄSinθ +λ/D

Here β is full width half maxima and Є is the r.m.s strain in the material.

23

Scanning electron microscopy

The scanning electron microscopy (SEM) is a powerful non-destructive technique to

study the topography, morphology and composition of the materials with much higher

resolution. When a beam of highly energetic electrons strikes the sample, the secondary

electrons, x-rays and back-scattered electrons are ejected from the sample. These electrons

are then collected by the detector and convert into signal that displays on a screen. In the

present study, the SEM micrograph was taken on the scanning electron microscope (JEOL-330

scanning microscope JEOL). As the samples are non-conducting, a thin layer of platinum is

coated using a sputter coater.

(Figure-2 Interaction of electron beam with specimen)

24

Dielectric Study

When a dielectric material is subjected to an external electric field, material becomes polarized

due to induced dipole and permanent moments. The polarization is directly proportional to the

macroscopic field i.e.

P =α E

Here α is the polarizabilty of atoms and molecules.

Types of polarizations are

1. Electronic polarization

2. Atomic or ionic polarization

3. Dipolar polarization

4. Interface or space charge polarization

(Figure-3 Frequency dependence of dielectric constant)

25

When a dielectric is subjected to the ac voltage, the electrical energy is absorbed by the

material and is dissipated in the form of heat. The dissipation is called dielectric loss (D).

In this work , to measure the relative permittivity (dielectric constant) and dielectric loss

PSM 1735 was used .The electroded samples were used to make the measurements. The PSM

was interfaced with the computer and the data (capacitance and D factor) was collected as a

function of temperature at different frequencies. The measured capacitance was then converted

into dielectric constant using the following formula:

C= εo ε r A / d

ε r =C d/ εo A

Where, C: Capacitance in farad (F)

ε: Permittivity of free space in farad per meter (8.85 x 10 -12 F/m)

ε r : Dielectric constant or relative permittivity of the sample.

A : Area of each plane electrode in square meters (m 2 )

d : Separation between the electrodes in meters (m)

In the present study, we have taken data using PSM-1735 impedance analyzer.

26

Chapter-3

27

Result and Discussion

3.1 Structural analysis

(Figure.1 X-ray diffraction pattern of Pb(1-x)Lax(Fe0.5Nb0.5)(1-x/4)O3 (x=0.00,0.02,0.04,0.06,0.08)

at room temperature )

Figure 1 compares the room temperature XRD patterns of calcined powder of Pb(1-x)Lax(Fe0.5

Nb0.5)(1-x/4)O3 (x=0.00, 0.02, 0.04, 0.06, 0.08). The diffraction pattern is different from that of

ingredients suggest the formation of new compounds with a presence of small amount of

secondary Phase (around the peak position of 27o). It has also been observed that, there is a

20 40 60 80

x=0.08

x=0.06

x=0.04

x=0.02

x=0.00

Intensity (A.U)

2 (degree)

Pb(1-x)

Lax(Fe

0.5Nb

0.5)(1-x/4)

O3

28

change in the peak position, peak intensity and peak shape of PFN on increasing La

concentration. The peak position full width at half maximum FWHM (β), and intensity of each

peak were calculated using commercially available software (PEAK FIT). Indexing of XRD

patterns was carried out using diffraction angle (2θ) and intensity value of each peak by a

standard IUCR software CHECK-CELL [ 15]. The best agreement in observed and calculated 2ө

(i.e., Δθ (2θobs-2 cal) = minimum) for monoclinic system (Table-1). The crystal structure was

found to be monoclinic for all compositions. The lattice parameters, unit cell volume, of the

samples are listed (Table-2).The lattice parameters were observed to be decrease with increase in

La concentration.

Table-1 Comparison of observed and calculated 2θ-values, miller indices of La+3

modifies Lead iron niobate.

Pb(1-x) Lax (Fe0.5 Nb 0.5)(1-x/4)O3 x=0.00

Sl

No.

2θ(observed)

degree

2θ(calculated)

degree

2θ

difference

h k l

1 22.1344 22.1357 -0.0013 0 0 1

2 31.5138 31.4979 0.0159 2 0 0

3 38.8482 38.8370 0.0112 0 2 1

4 45.1709 45.1561 0.0148 0 0 2

5 50.7530 50.8009 -0.0479 -1 1 2

6 56.1055 56.0951 0.0104 0 2 2

7 65.8126 65.8299 -0.0173 2 2 2

8 70.3305 70.3272 0.0033 0 0 3

9 74.7229 74.7076 0.0153 -1 1 3

29

Pb(1-x) Lax (Fe0.5 Nb 0.5)(1-x/4)O3 x=0.02

Sl

No.

2θ(observed)

degree

2θ(calculated)

degree

2θ

difference

h k l

1 22.2348 22.1780 0.0568 -1 1 0

2 31.6164 31.5551 0.0613 0 2 0

3 38.9645 38.9607 0.0038 2 0 1

4 45.2759 45.2461 0.0298 -2 2 0

5 50.9582 50.9709 -0.0127 2 2 1

6 56.2217 56.1961 0.0256 1 3 1

7 65.9011 65.8863 0.0148 0 4 0

8 70.4006 70.4054 -0.0048 1 3 2

9 74.8287 74.9018 -0.0731 -2 4 0

Pb(1-x) Lax (Fe0.5 Nb 0.5)(1-x/4)O3 x=0.04

Sl

No.

2θ(observed)

degree

2θ(calculated)

degree

2θ

difference

h k l

1 22.0875 22.1322 -0.0447 0 0 1

2 31.5523 31.5505 0.0018 0 2 0

3 38.7717 38.8164 -0.0447 -2 0 1

4 45.2336 45.2003 0.0333 -2 2 0

5 50.9489 50.9088 0.0401 2 2 1

6 56.1892 56.1764 0.0128 1 3 1

7 65.8144 65.8506 -0.0362 2 2 2

8 70.4006 70.4254 -0.0248 0 4 1

9 74.7496 74.7166 0.0330 -1 1 3

Pb(1-x) Lax (Fe0.5 Nb 0.5)(1-x/4)O3 x=0.06

Sl

No.

2θ(observed)

degree

2θ(calculated)

degree

2θ

difference

h k l

1 22.1666 22.1807 -0.0141 -1 1 0

2 31.5940 31.5574 0.0366 0 2 0

3 38.8508 38.8773 -0.0265 0 2 1

4 45.2734 45.2519 0.0215 -2 2 0

5 50.8698 50.8452 0.0246 1 1 2

6 56.2343 56.1815 0.0528 1 3 1

7 65.8144 65.8374 -0.0230 2 2 2

8 70.4006 70.4371 -0.0365 0 4 1

9 74.9078 74.9162 -0.0084 3 3 1

30

Pb(1-x) Lax (Fe0.5 Nb 0.5)(1-x/4)O3 x=0.08

Sl

No.

2θ(observed)

degree

2θ(calculated)

degree

2θ

difference

h k l

1 22.0875 22.0962 -0.0087 0 0 1

2 31.5556 31.5483 0.0073 0 2 0

3 38.7717 38.7849 -0.0132 -2 0 1

4 45.2455 45.2171 0.0284 -2 2 0

5 50.8698 50.8322 0.0376 1 1 2

6 56.1676 56.1725 -0.0049 1 3 1

7 65.8144 65.8465 -0.0321 2 2 2

8 70.4006 70.4061 -0.0055 0 4 1

9 74.9078 74.9054 0.0024 3 3 1

The broadening in the X-ray line profile is mainly due to small crystallite size and anisotropic

strain. Since both the effects are independent of each other we can separate out by Williamson–

Hall method.

(Figure2.Williamson–Hall plot of PFN) (Figure3. crystallite size dependence on x)

0.02 0.04 0.06 0.08 0.10

0.2

0.4

0.6

0.8

(-1 1 3)

(2 2 2)(0 2 2)

(0 0 2)

Pb(1-x)Lax(Fe0.5Nb0.5)(1-x/4)O3

x=0.00

cos

Crystallite size=1988 A0(27)

sin

(0 2 1)

0.00 0.02 0.04 0.06 0.08

500

1000

1500

2000

Crystallite size (Ao)

x

31

Williamson-Hall equation

β Cosθ= 4ЄSinθ +λ/D

Here D crystallite size, λ wavelength used Є r.m.s strain in the samples.

By plotting βcos θ vs. sinθ. r. m. s. strain can be calculated from the slope and the crystallite size

can calculated from the ordinate intercept Crystallite size decreases with increase in La

incorporation at Pb site of PFN.

Table-2

Composition

(x)

a(Ao) b(Ao) c(Ao) α=

γ

β

(degree)

Crystalli

te size

(Ao)

0.00 5.6805(44) 5.6800(47) 4.0157(09) 90 90.14 1988

0.02 5.6764(20) 5.6704(30) 4.0206(15) 90 90.13 1039

0.04 5.6764(63) 5.6712(29) 4.0163(22) 90 90.11 914

0.06 5.6754(84) 5.6700(27) 4.0179(63) 90 90.02 891

0.08 5.6721(33) 5.6716(17) 4.0228(23) 90 90.20 724

32

3.2 Scanning electron microscopy (SEM)

(Figure4 Room temperature SEM micrographs of Pb(1-x)Lax(Fe0.5Nb0.5)(1-x/4)O3

(x=0.00,0.04,0.06,0.08) )

The SEM micrographs show the polycrystalline nature of microstructure where grain sizes are

inhomogeneously distributed throughout the sample with certain degree of porosity. The grain

and grain boundaries are clearly distinct. The average grain size decreases with increase in La

concentration at Pb site as observed by visual examination. The average grain size distributions

of the samples were found to be 7μm to 3μm.

33

3.3 Dielectric Study

Figure 5. Shows the temperature dependence of dielectric constant for La modified PFN

ceramics from room temperature to 250oC at few selected frequencies with oscillation amplitude

of 1V. It is observed that εr decreases monotonically on increasing frequency at all the

temperatures, which represents the behavior of polar dielectric materials. It can be seen from the

graph that εr increases with increase in temperature, attains its maximum value (εmax) and then

decreases. This observed dielectric anomaly shifts toward lower temperature with increase in La

concentration from x=0.00 to 0.02 and falls below the room temperature for x=0.04, 0.06, 0.08.

This dielectric anomaly is observed for La-modified PFN represents the ferroelectric –

paraelectric phase transition which is diffuse type. It is also observed that dielectric constant

decreases with increase in La substitution at Pb site which may be due to turning off of

polarizibility of Pb in PFN. The above observation is related to the decrease of grain size due to

the La3+

ions doping as observed from SEM. It is observed that for all the samples tanδ

increases on increasing temperature with one anomaly in the temperature range 50-200oC .This

anomaly in tanδ shifting towards the high temperature side and the broadening of the peak

increases with increase in frequency The reason for increase in tanδ at high temperature (in

ceramics) can be attributed to the space charge polarization .Rapid increase in tan D at high

temperatures is attributed to increase in electrical conductivity.

34

50 100 150 200 250

1000

2000

3000

4000

10KHz

50 KHz

100 KHz

500 KHz

1MHz

Temperture(oC)

r

x=0.02

Figure5.Temperture dependent dielectric constant for Pb(1-x)Lax(Fe0.5Nb0.5)(1-x/4)O3

(x=0.00,0.02,0.04,0.06,0.08)

50 100 150 200 250

1000

2000

3000

4000

10KHz

50KHz

100KHz

500KHz

1MHz

x=0.08

Temperture(oC)

r

50 100 150 200 250

1000

2000

3000

4000

5000

6000

1KHz

5kHz

10KHz

50KHz

100KHz

500kHz

1MHz

Temperature (0C)

r

x=0.0

50 100 150 200 250

1000

2000

3000

4000 10KhZ

50KHz

100Khz

500KHz

1MHz

x=0.06

Temperture(oC)

50 100 150 200 250

1000

2000

3000

Temperture(oC)

r

10KHz

50kHz

100KHz

500KHz

1MHz

x=0.04

35

Figure6.Temperture dependence of dielectric loss (tanδ) for Pb(1-x)Lax(Fe0.5Nb0.5)(1-x/4)O3

(x=0.00,0.02,0.04,0.06,0.08)

50 100 150 200 250

0.0

0.2

0.4

0.6

tan

Temperture(oC)

x=0.02

50 100 150 200 250

0.0

0.2

0.4

0.6

10KHz

50KHzz

100KHz

500KHz

1MHz

tan

Temperture(oC)

x=0.04

50 100 150 200 250

0.0

0.2

0.4

0.6

10KHz

50KHz

100KHz

500KHz

1MHz

tan

x=0.06

50 100 150 200 250

0.0

0.2

0.4

0.6

10KHz

50KHz

100KHz

500KHz

1MHz

x=0.08

tan

Temperture(oC)

36

Conductivity studies

Figure 7. Shows the variation of ac electrical conductivity (σac) of PFN as a function of

frequency at different temperatures and for all La modified PFN ceramics. The ac

conductivity was calculated using the relation σac = εoεrω tan δ. In the low frequency region, ac

conductivity remains almost constant (i.e. frequency independent plateau region, representing d.c

conductivity) whereas the dispersion of conductivity was observed in the higher frequency

region. The crossover from the frequency independent region to the frequency dependent regions

represents the onset of the conductivity relaxation, indicating the transition from long

range hopping to the short-range ionic motion in the material. The frequency dependence of ac

conductivity obeys Jonscher’s power law, i.e. σac = σ0 + Aωn , where σ0 is frequency

independent conductivity (which is related to dc conductivity), A is the temperature dependent

pre-exponential factor and n is frequency exponent, (0 < n < 1). As ac conductivity

increases with rise in temperature, all the compounds have negative temperature coefficient of

resistance (NTCR) behavior typically semiconducting behavior.

37

Figure7. Variation of σac with frequency for Pb(1-x)Lax(Fe0.5Nb0.5)(1-x/4)O3

(x=0.00,0.02,0.04,0.06,0.08)

0.1 1 10 100 1000

1E-6

1E-5

1E-4

1E-3

0.01

ac (

m)

250C

500C

750C

1000C

1280C

1520C

1770C

2020C

2270C

2520C

2770C

PFN x=0.00

Frequency (KHz)0.1 1 10 100 1000

1E-5

1E-4

1E-3

0.01

30 oC

55 oC

80 oC

105 oC

128 oC

152 oC

177 oC

202 oC

227 oC

257 oC

281 oC

301 oC

ac (

m)

Frequency(KHz)

PFN x=0.02

0.1 1 10 100 1000

1E-5

1E-4

1E-3

0.01

ac (

m)

PFN X=0.04

Frequency (Hz)

300C

550C

850C

1050C

1330C

1570C

1820C

2070C

2320C

2570C

2810C

3060C

0.1 1 10 100 1000

1E-5

1E-4

1E-3

0.01

ac (

m)

PFN X=0.06

Frequency (KHz)

300C

550C

850C

1050C

1330C

1570C

1820C

2070C

2320C

2570C

2810C

3060C

0.1 1 10 100 1000

1E-5

1E-4

1E-3

0.01

Frequency(KHz)

ac (

m)

30 oC

55 oC

80 oC

105 oC

128 oC

157 oC

182 oC

207 oC

232 oC

257 oC

281 oC

306 oC

PFN x=0.08

38

3.4. Impedance Spectroscopic studies

Figure 8. represents the temperature dependent complex impedance (Nyquist plot) of La-

modified PFN ceramics. The linear variation of Z// with Z

/ in the complex impedance plot in low

temperature range from room temperature to 200◦C indicates the insulating properties of the

material. Above 200 ◦C, circular arc formation trend started which is due to the increase of

conductivity. The impedance plots seem to have two overlapped semicircles. Each semicircle of

the Nyquist plot corresponds to the different contribution to the electrical response. The high

frequency semicircle can be attributed to the bulk (grain) property and low range frequency

corresponds to grain boundary property of the material .The relaxation process associated with

this observation is non-ideal in nature. This origin of non-Debye type behavior may be due to

several factors such as grain orientation, grain size distribution, grain boundaries, atomic defect

distribution and stress–strain phenomena. The intercept of the semicircular arc on the real axis

gives the dc resistance of the material. It is seen from that the dc resistance increases with

increase in the La content.

39

Figure8. Nyquist plot for Pb(1-x) Lax (Fe0.5 Nb 0.5)(1-x/4)O3 (x=0.00,0.02,0.04,0.06,0.08)

0 2 4 6 8

0

2

4

6

8

Z' (K

Z" (K

2070C

2320C

2570C

2820C

3060C

PFN x=0.06

0 1 2 3 4 5

0

1

2

3

4

5

Z' (K

Z" (K

2070C

2320C

2570C

2820C

3060C

PFN x=0.04

0 2 4 6 8

0

2

4

6

8 Z" (K

2020C

2270C

2520C

2720C

PFN x=0.00

Z' (K 0 2 4 6 8

0

2

4

6

8 301

oC

281 oC

252 oC

222 oC

202 oC

Z'' (K

)

Z' (K )

PFN x=0.02

0 2 4 6 8 10

0

2

4

6

8

10

Z'(K )

207 oC

232 oC

257 oC

281 oC

306 oC

PFN x=0.08

Z''(K

)

40

Conclusions

In the present investigation, we have prepared the La+3 modified lead iron niobate perovskite

ceramics having the general chemical formula Pb(1-x) Lax (Fe0.5 Nb 0.5)(1-x/4)O3 (x=0.00, 0.02,

0.04, 0.06, 0.08) using high temperature solid state reaction route. The structural (XRD),

microstructural (SEM) and electrical (dielectric and impedance) properties of the proposed

compounds have been studied extensively.

Based on the results obtained following conclusions have been made.

(1) PFN and La-modified PFN samples were prepared by mixed oxide high temperature

solid-state reaction route.

(2) X-ray diffraction (XRD) studies confirmed the formation of the compounds with

monoclinic crystal system. Both the lattice parameters and crystallite size decreases with

increase in La+3

concentrations.

(3) Scanning electron micrographs (SEM) of the compounds showed (1) polycrystalline

nature of microstructure, (2) decrease in grain size with increase in La+3

concentration,

(3) uniform distribution of grain size with high density.

(4) The ferroelectric to paraelectric phase transition temperature was found to be 115oC for

PFN and decreases with increase in La+3

substitution. For high concentration (i.e.,

x=0.04, 0.06 and 0.08) the phase transition temperature falls below the room temperature.

(5) The ac conductivity of the La-modified PFN obeyed the Jonsher’s power law behavior.

(6) Complex impedance spectroscopy method has been used for better understanding of

relaxation process and to establish relationship between the microstructure–electrical

properties of the compounds.

41

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